Properties of the bacteriophage alpha 3 mutants with deletion and/or insertion in the complementary strand origin

Bacteriophage alpha 3 origin of complementary strand DNA synthesis (-ori) contains two potential secondary loop structures (I and II), which have been implicated in direct recognition sites for host Escherichia coli dnaG protein. We have introduced nucleotide deletion or insertion within the -ori region, by nuclease digestion and polymerase treatment of alpha 3 replicative form DNA. Deletion mutants (delo) showing the following in vivo properties were isolated: minute plaque size, longer latent period, and reduction both in phage yield and viral DNA synthesis as compared with the wild-type phage. In addition, several strains among the delo mutants did not grow on host E. coli dnaB cells at 42 degrees C. These results suggest that certain type of deletion in the -ori region converts simple priming of the complementary strand synthesis to a more complicated phi X174-type initiation that depends on the primosome, rather than on dnaG protein alone.